Nanoparticles Change Crystal Structure When They Get Wet, Research Shows

Molecular simulations predict a change from a more distorted to a more periodic structure accompanying binding of water molecules to a nanoparticle surface.Credit: H. Zhang, B. Gilbert, F. Huang, J. Banfield, University of California at Berkeley.
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Arlington, Va.—As scientists shrink materials down to the nanometer scale, creating nanodots, nanoparticles, nanorods and nanotubes a few tens of atoms across, they've found puzzling behaviors that fire the imagination and promise unforeseen applications.

Now, in a paper appearing in the August 28th issue of Nature, a team of scientists at the University of California, Berkeley, report an unusual effect that could have both good and bad implications for the development of semiconductor devices at the nanometer scale.

The discovery, which was supported by the National Science Foundation (NSF), also could provide a way to tell whether space rocks came from planets that contain water.

NSF is an independent federal agency that supports fundamental research and education across all fields of science and engineering.

According to the report, miniscule clumps containing only 700 or so molecules of the semiconducting material, zinc sulphide (ZnS), become more ordered and closer to the structure of bulk ZnS when the particles are exposed to water.

"This important study shows how nanoparticles can respond to their surrounding environment, opening up the possibility of developing less invasive nanosensors," said David Lambert, program director in NSF's division of earth sciences. "The study also illustrates very dramatically our abilities to probe and understand matter at the nanoscale by combining advanced instrumentation and sophisticated mathematical modeling."

People had noticed that "nanoparticles often had unexpected crystal structures and guessed it might be due to effects on the particle's surface," said physicist Benjamin Gilbert of UC Berkeley. "This is a clear-cut demonstration that surface effects are important in nanoparticles."

Gilbert and co-author Hengzhong Zhang, suggest that many types of nanoparticles may be as sensitive to water as ZnS is. "We think that, for some systems of small nanoparticles maybe 2-3 nanometers across, this kind of structural transition may be common," Zhang said.

"There's a good and bad side to this," Gilbert added. "If we can control the structure of a nanoparticle through its surface, we can expect to produce a range of structures depending on what molecule is bound to the surface. But this also produces unexpected effects researchers may not want."

Such surface effects could have implications for understanding extraterrestrial materials. A nanoparticle that formed in a place with water, such as Earth, would have a more ordered surface than would a nanoparticle formed in dry outer space. Team leader Jillian Banfield, an earth and planetary scientist, has been looking at microscopic and nanoscale particles in rocks, minerals and the environment to determine what information the tiny bits can provide about their origin.

Some microbes, for example, produce nanoparticles—such as ZnS in the form of a mineral called sphalerite, iron oxide and uranium oxide—as a byproduct of metabolism. The trick is to distinguish these biogenic nanoparticles from similar nanoparticles formed by geologic processes.

After modeling and constructing ZnS nanoparticles three nanometers across, the researchers using synchrotron x-ray techniques to observed changes in the particles' crystal structure when exposed to different liquids. In methanol, the particles developed a disordered surface but the core remained similar to bulk ZnS. When they added water to the methanol, the scientists observed a much more ordered structure to the particles' surface. When the dry nanoparticles were again immersed in methanol, they reverted to their original structure.

"This result demonstrates that these nanoparticles can respond to changes in their surface environments," Banfield said.

"To our knowledge, these are the first surface-driven room temperature transitions observed in nanoparticles," she added. "If substances can be found that stabilize alternative variants, there may be ways to generate uncommon structures through surface binding after the nanoparticle is synthesized."

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